Field of the Invention
The invention relates to a multilayer structure comprising a base layer comprising at least one transparent thermoplastic and a specific inorganic infra-red absorber as well as an IR-reflecting multi-ply layer, the production of such a multilayer structure and the use thereof for the production of glazing made of plastic in buildings, automobiles, rail vehicles and aircraft.
Description of Related Art
Glazing produced from compositions comprising transparent thermoplastic polymers, such as e.g. polycarbonate, offers many advantages over conventional glazing of glass for the vehicle sector and for buildings. These include e.g. increased fracture-proof properties or saving in weight, which in the case of automobile glazing makes possible a higher safety of passengers in the event of traffic accidents and lower fuel consumption. Finally, transparent materials which comprise transparent thermoplastic polymers allow a considerably greater freedom of design due to the simpler formability.
A disadvantage is, however, that the high thermal transmission (i.e. transparency to IR radiation) of transparent thermoplastic polymers in sunlight leads to an undesirable heating inside vehicles and buildings. The increased temperatures in the inside reduce the comfort for the passengers or occupants and can result in increased demands on the air-conditioning, which in turn increase energy consumption and in this way cancel out the positive effects again. In order nevertheless to take into account the requirement of a low energy consumption combined with a high passenger or occupant comfort, panes which are equipped with appropriate heat protection are necessary.
As has been known for a long time, the majority of solar energy falls to the range of the near infra-red (NIR) between 750 nm and 2500 nm, in addition to the visible range of light between 400 nm and 750 nm. Penetrating solar radiation e.g. is absorbed inside an automobile and emitted as long wavelength thermal radiation having a wavelength of from 5 μm to 15 μm. Since in this range conventional glazing materials—in particular thermoplastic polymers which are transparent in the visible range—are not transparent, the thermal radiation cannot radiate outwards. A greenhouse effect is obtained and the interior heats up. In order to keep this effect as low as possible, the transmission of the glazing in the NIR should therefore be minimized as far as possible. Conventional transparent thermoplastic polymers, such as e.g. polycarbonate, however, are transparent both in the visible range and in the NIR.
Additives which have the lowest possible transparency in the NIR without adversely influencing the transparency in the visible range of the spectrum e.g. are therefore required.
In order to impart to the plastics heat-absorbing properties, corresponding infra-red absorbers (IR absorbers) are therefore employed as additives. In particular, IR absorber systems which have a broad absorption spectrum in the NIR range with a simultaneously low absorption in the visible range (low intrinsic colour) are of interest for this. The corresponding plastics compositions should moreover have a high heat stability and an excellent light stability.
A large number of IR absorbers based on organic or inorganic materials which can be employed in transparent thermoplastics are known. A selection of such materials is described e.g. in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), in U.S. Pat. No. 5,712,332 or JP-A 06240146.
Nevertheless, IR-absorbing additives based on organic materials often have the disadvantage that they have a low stability towards exposure to heat or irradiation. Thus, many of these additives are not sufficiently stable to heat to be able to be incorporated into transparent thermoplastics, since temperatures up to 330° C. are required during processing of the plastics. Furthermore, the glazing is often exposed to temperatures of more than 50° C. over relatively long periods of time during use, due to the solar irradiation, which can lead to decomposition or to degradation of the organic absorbents. Furthermore, the organic IR absorbers often do not have sufficiently broad absorption bands in the NIR region, so that their use as IR absorbers in glazing materials is inefficient. An undesirable, intense intrinsic colour of these systems often also additionally occurs.
IR-absorbing additives based on inorganic materials are often significantly more stable compared with organic additives. The use of these systems is also often more economical, since in most cases they have a significantly more favourable price/performance ratio. Thus, materials based on finely divided borides, such as e.g. lanthanum hexaboride, have proved to be efficient IR absorbers, since they have a broad absorption band combined with a high heat stability. Such borides based on La, Ce, Pr, Nd, Tb, Dy, Ho, Y, Sm, Eu, Er, Tm, Yb, Lu, Sr, Ti, Zr, Hf, V, Ta, Cr, Mo, W and Ca are described e.g. in DE-A 10 392 543 or EP-A 1 559 743.
However, their significant intrinsic colour is a disadvantage of these additives. After incorporation, the boride-containing additives impart to the transparent plastic a characteristic green coloration, which is often undesirable since it severely limits the margin for imparting a neutral colour.
For example, DE-A 10 392 543, US-A 2004/0028920, EP-A 1 559 743, EP-A 1 865 027 and EP-A 2 009 057 describe polymer compositions based on inorganic boride particles. However, these compositions have the undesirable, significant intrinsic colour mentioned.
EP-A 1 559 743 describes polycarbonate compositions comprising inorganic IR absorbers based on borides in combination with UV absorbers. However, these compositions do not meet the high requirements with respect to a low energy transmission, in particular a low secondary energy transmission. Furthermore, these compositions have a significant intrinsic colour.
To compensate this intrinsic colour, relatively large amounts of further, preferably complementary colouring agents are often employed, but this impairs the optical properties of the composition and leads to a significantly reduced transmission in the visible range. This is undesirable especially in vehicle glazing, or is inadmissible in specific cases where the vision of the driver must not be impaired.
IR-absorbing additives from the group of tungsten compounds which have a lower intrinsic absorption in the visible spectral range compared with the inorganic boride-based IR absorbers known from the prior art are furthermore known. The preparation and the use of these substances in thermoplastic materials is described, for example, in H. Takeda, K. Adachi, J. Am. Ceram. Soc. 90, 4059-4061, (2007), WO-A 2005/037932 and WO-A 2009/059901. However, the lack of long-term stability to exposure to heat has proved to be a disadvantage. While the instability of tungsten oxides to heat is known per se and has been described, for example, in Romanyuk et al.; J. Phys. Chem. C 2008, 112, 11090-11092, it has been found that when these compounds are incorporated into a polymer matrix, the absorption in the IR range also decreases significantly during thermal storage of the corresponding polymer compositions, such as e.g. in a polycarbonate composition, at elevated temperature.
For use in the glazing sector, in particular for automobile glazing, however, it is absolutely essential that the corresponding IR-absorbing plastics compositions have a long-term stability to higher temperatures, in particular those temperatures which an article of plastic can assume under intensive solar irradiation (e.g. 50° C.-110° C.). It must furthermore be ensured that the composition can be processed under conventional process conditions, without the IR-absorbing properties already being reduced as a result.
A further disadvantage of glazing containing IR absorbers is its storage of heat. A pane containing an IR absorber heats up when irradiated with sunlight and the heat stored is thereby afterwards released partly again to the outside, but partly also into the inside of the vehicle or building. This secondary heat transfer into the inside is critical, because this heats up the inside in addition to the direct energy transmission. In order also to take into account the secondary heat transfer, the so-called “total solar energy transmitted to the inside of a glazing”, in the following also “total solar transmittance” TTS according to ISO 13837, is often stated for the performance of a system. A system with the highest possible transmission (Ty) in the visible range having a low primary and secondary energy transmission (TTS) is often aimed for.
To avoid the indirect heating up by the secondary heat transfer, pigments or coatings which reflect the infra-red radiation are known. As a result of this reflection, the pane heats up less and the secondary heat transfer to the inside is lower. However, such systems often have a transmission which is too low for panes. These panes are often only translucent. Other disadvantages can lie e.g. in the shielding from radio waves and can thus impair the function of mobile communications and navigation equipment.
U.S. Pat. Nos. 6,333,084 and 5,589,280 disclose e.g. multilayer structures which comprise, inter alia, IR-reflecting thin layers of metal. However, these systems impede the passage of radio waves and therefore interfere with, inter alia, navigation or mobile communications equipment. Furthermore, thin layers of metal may be susceptible to corrosion.
DE-A 10 117 786 describes multilayer structures of a base layer comprising organic IR absorbers and an IR reflection layer. Merely the organic IR absorbers alone, however, have the above-mentioned stability problems.
US-A 2006/0251996 discloses multilayer sheets comprising a core layer comprising a thermoplastic polymer and a metal oxide as an IR-absorbing additive. These systems also do not meet the high requirements with respect to a low energy transmission, in particular a low secondary energy transmission. WO-A 99/36257 and US-A 2004/0032658 disclose multilayer systems which have IR-reflecting properties. However, these systems do not meet the high requirements with respect to an extremely low energy transmission combined with a high visual light transmission.
US 2008/0292820 A1 describes a multilayer film which comprises nanoparticles based on metal oxides, such as indium tin oxide (ITO) and/or antimony oxide (ATO), and IR absorbers, such as lanthanum hexaboride. Due to the small thickness of the IR-absorbing layer, the multilayer structure nevertheless has the disadvantage that different depths of colour may occur in the outer and inner regions of the film.
US 2008/0075948 A1 describes a multilayer structure consisting, inter alia, of an IR-reflecting layer and an IR-absorbing layer. Due to the structure of the multilayer body and the IR absorbers used, no uniform colour flow over the entire region of the film is to be expected.
WO 2004/000549 A1 describes a multilayer body of laminated glass. The structure comprises, inter alia, an IR-reflecting layer and an IR-absorbing layer. Here also, due to the small thickness of the IR-absorbing layer and the IR absorber used, no uniform colour flow is to be expected.
The combination of multi-ply layers with an adjacent layer containing IR absorber is described by way of example in US-A 2008/0291541 and US-A 2006/0154049. The layer containing IR absorber is preferably a thin coating of a maximum of 50 μm. Disadvantages of the structure described are the high intrinsic colour of the absorber layer and accompanying very low light transmission in the visible wavelength range. The thin layers containing IR absorber furthermore have a colour flow, since slight deviations in the thickness severely impair the colour.
There therefore continued to be a need for a multilayer structure of plastic which has a high transmission (Ty) in the visible range and a low primary and secondary energy transmission (TTS), without the above-mentioned disadvantages occurring.